Rotary axes for modern day machine tools must exhibit increasingly accurate and reliable rotary control and effective torsional dynamic stiffness. Incorporation of electronically driven servo axes for the drive arrangement of a machine tool spindle or C-axis drive system is one preferred manner for providing capabilities of improved accuracy and control. In order to provide such a drive system exhibiting a predetermined torsional stiffness, the position and velocity loop gains of the system must be set at relatively high levels.
While dynamic stiffness generally increases with increased loop gains, at some point, the increased gains can cause instabilities in the system. Particularly, the inertia of the rotary axis coupled throuhh a direct drive train to a servo motor device can often exhibit instabilities which can adversely affect the performance of the machine tool. Such instabilities further limit the amount of loop gain and torsional stiffness which can be obtained on the system, and, consequently, such oscillation and/or vibration must be effectively eliminated from the servo axis.
Various torsional damping arrangements have been known and used in the industry for addressing oscillations of mechanically driven crankshafts and the like. For example, U. S. Pat. No. 2,585,382, which issued to R. Guernsey, describes a torsional vibration damper for a rotatably driven crankshaft wherein a damping mass is elastically situated for rotation relative to the crankshaft, and a second damping mass is housed within a viscous fluid. Viscoelastic shear between the first mass and a rubber annulus connecting that mass to the crankshaft provides a vibration damp, while oscillation of the rotating second mass causes shear between the mass and its viscous fluid support, thereby damping oscillatory movements. Tuning of this device can be affected by changing the elastic unit or by changing the amount of mass of the dampers.
Another damping device for a rotatable shaft is shown in U.S. Pat. No. 3,545,301, which issued to R. Richter. Similar to the Guernsey arrangement, the Richter stepping motor damper includes a strip of viscoelastic material wrapped around the shaft and held in a constant strain preload by the inside diameter of a damping mass attached to the rotatable output shaft. The polar mass moment of inertia of the damping mass is perferably chosen to be about 50% of that of the motor, and the stiffness of the damping element is determined so as to preferably be effective for the full variation in torsional natural frequency of the motor. This arrangement, however, is directed toward only the motor dynamics and does not address compound system of a motor, transmission and load.
Other torsional type damping devices have also been utilized, such as that shown in U.S. Pat. No. 2,346,732, which issued to J. Crawford, et al. The Crawford vibration damper comprises a drive plate formed as a disk of thin steel which is cut away to provide a peripheral rim and a plurality of radial spokes. This drive plate is made relatively thin so that its rim can follow a true rotational path while its hub may wobble. A dampener spider formed as a sheet metal disk having a discontinous rim and radial arms which attach to friction pads is provided. The friction pads bear against a continuous friction ring so that radial movement of the spider arms caused by wobbling of the drive plate hub is damped by friction.
A damper for vibration induced by tool and cutter impacts (chatter) in rotary tool supporting members is shown in U.S. Pat. No. 2,714,823, which issued to A. Dall, et al. Like the Guernsey torsional vibration damper discussed above, the Dall damper includes a heavy leas weight or inertia member rotatably housed within a chamber wherein springs help develop a predetermined friction between the weight and its surrounding walls. The Dall, et al damping disk is subject to both relative rotary and radial movement, and because the vibration impulse is transferred through a broad band type frictional connection, a time delay causes the momentum in the weight to oppose vibration forces, resulting in relative damping.
Similarly, a vibration dampener attached to a crankshaft pulley is shown in U.S. Pat. No. 2,346,972, which issued to F. Kishline. Particularly, the Kishline vibration dampener includes a wheel with a flat disk-shaped portion fixed to a crankshaft by a key, wherein the forward face of the wheel has a series of pockets each containing a cylindrical inertia member. These inertia members oppose acceleration and deceleration forces in a manner out-of-phase with such forces, thereby offsetting and canceling vibration and oscillation forces in the rotating shaft.
It is also known that servo systems perform erratically when output transducers resonate at frequencies within the operating bandwidth of the system. To this end, conventional wisdom indicates that a compensating network, such as a bridged-T, be contained within the system loop to insure stability of servo systems containing resonating output transducers. In such networks, resistors and capacitors are variously utilized to offset gain and phase characteristics of the transducers. Damping ratios and natural frequencies are determined by calculations to meet the requirements for stable operation. Precise cancellation of the poles is difficult, but has been found to be unnecessary for stable operation. High precision components, however, must be utilized, and effects of aging and operating conditions such as temperature must be considered in selecting components to maximize potential for continued system stability.
It is also recognized that instabilities in these systems can be addressed by lowering the servo loop gains, but, as indicated above, rotary axis stiffness is also forfeited in that process. Consequently, typically, resonances in servo systems are most often minimized by incorporation of a compensating network in the servo control to filter or offset such resonance.
Despite the various structural devices known for damping oscillations of mechanically driven shafts, and the relatively sensitive high precision filter-type compensating networks for servo systems, there has not been previously available a relatively simply mechanical arrangement for dependably damping vibrations and oscillations of an electronically driven servo axis. Instabilities resulting from the relatively high position and velocity loop gains required to provide certain predetermined torsional stiffness for increased accuracy and reliability created serious limitations and/or problems in many applications. A simple and tunable dynamic damper capable of minimizing servo oscillations and vibrations while allowing optimum position and velocity loop gains and torsional stiffness in a stable manner was needed.